Selecting text with Shift+Curtsor Down works "normally" (the text in the current line that is behind the current position of the cursor gets selected, and the part of the text in the next line up to the position of the cursor is added o the selection. But if I press Shift + Cursor Up, only the current line is selected and it gets deselected immediately if I keep selecting text.

It's a bit hard to explain, here's a gif:

in the first part I select the text wirth Shift+Down, but in the second part watch what happens with Shift+Up.

I tried this on both my desktop and laptop and it glitches on both. btw on my (not super powerful but solid) laptop the scrolling of text in IDE slows down horribly if I try to load a file like Hollyman.